The disclosures herein relate generally to deep water pipeline construction and more particularly to a method and apparatus for manufacturing a double walled pipeline.
Double-walled pipe, also called pipe-in-pipe, is often used in deep water pipeline construction and includes an inner pipe positioned within an outer pipe in order to provide a number of advantages such as, for example, providing thermal insulation to the oil flow. Adding new pipe sections to a double-walled pipe is typically accomplished by providing an add-on section with an inner pipe protruding from one of its ends. The inner pipe of the add-on section is welded to the inner pipe of the pipeline. After completion of the inner pipe welding, the outer pipe of the add-on section is slid over the inner pipe until it abuts the outer pipe of the pipeline. The outer pipe welds are then made.
Bulkheads may be used to couple sections of the double-walled pipe together, as well as to couple the double-walled pipe to pipeline components such as, for example, wells, sub-sea manifolds, and pipeline terminations. However, when the add-on section contains a bulkhead the procedure described above is not possible, as the outer pipe cannot slide over the inner pipe. The typical procedure for an add-on section with a bulkhead is to first weld the protruding inner pipe of the add-on section to the inner pipe of the pipeline and then close the remaining opening between the outer pipes of the add-on section and the pipeline with half-shells. The length of the half-shells is chosen such that predetermined stress distributions are achieved in the inner and outer pipes.
The coupling of the double-walled pipe with half-shells raises a number of issues. In order to achieve the desired stresses in the inner and outer pipes, the half-shells often require a significant length, which makes it difficult to achieve high quality welds and results in longer weld times. The half shells also require separate welding procedures, which increases cost and produces welds that are extremely sensitive to fatigue damage.
Accordingly, it would be desirable to provide a method and apparatus for manufacturing a double walled pipeline absent the disadvantages found in the prior methods discussed above.